Gita Series – 97: Bhagavad Gita Chapter VIII. Verses 23– 26:

Arjuna, I shall now tell you the different paths pursued by yogis at the time of leaving their bodies, one to return and another, not to return. Self realised persons, at the time of relinquishing their physical body, follow the path of fire, light, day time, bright half of the lunar month, six month period of northern course of the sun and go to the Brahman. They are led to the Brahman by these presiding deities. The other path is where yogis leave their bodies during smoke(smoke is generated due to improper fire), night, the dark fortnight of the lunar month, six month period of southern course of the sun and are led by the presiding deities of the above and reach the moon. After exhausting the effects of their good karmas, they come back. The two paths, the bright one and the dark one are eternal. Proceeding through the first of them, the yogi reaches the supreme state never to return. Proceeding by the other one, he returns to the mortal world.”
There are two type of interpretations for these stanzas. General interpretations talk about time factor like day time and night time, bright and dark lunar fortnights and northern course and southern course of the Sun. According to this interpretation, if one dies during bright time, he reaches the heavens and if one dies during darkness, he reaches the hell. As the time of death in not in our hands, these verses of Krishna cannot be interpreted on the face of it, but one has to look deep into these verses. It is also said that those who leave the mortal world during brightness reach the heavens, where gods live and those who leave during darkness reach the world of ancestors at Moon, to be born again.
There are two paths after death. One is to merge with the Lord and another is to come back to earth again, after experiencing the reward for his past deeds. The latter refers to the sojourn of the soul in the higher planes of the cosmos and begins its transmigration again. These verses also say that one presiding deity hands over the soul to another deity till the destination of the soul is reached. If an attempt is made to expand the significance of exercising control over the soul one deity after another, it could mean gradual liberation or krama mukti. His liberation becomes gradual, but steady. It beings with the least powerful light to the most powerful Self illuminating light. This is the path of no-return where the soul becomes one with the Brahman.
The second path is the path of darkness which leads to the world of ancestors. This world is presided by the Moon. Moon always refers to sensual pleasures, which means that the soul has not been totally purified yet, to become fit enough to merge with the Brahman. His spiritual path is not yet complete, though he has made considerable spiritual progress. As a result of this considerable spiritual progress he reaches the Moon, continues to enjoy there and takes another birth to continue his spiritual journey from where he had left during his last life. He perfects himself during further births and proceeds towards the path of light for liberation. But a perfected yogi goes through the illuminated path in stages to ultimately merge with the Brahman.
The choice of the path is decided on the basis of one’s karmic account. Brightness indicates knowledge and darkness indicates ignorance. Brightness leads to the Brahman and darkness leads to transmigrations. The concepts discussed in these verses are explained in great detail in Praśna Upaniṣad.
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